Very low shot noise in carbon nanotubes

We have performed noise measurements on suspended ropes of single wall carbon nanotubes (SWNT) between 1 and 300 K for different values of dc current through the ropes. We find that the shot noise is suppressed by more than a factor 100 compared to the full shot noise 2eI. We have also measured an individual SWNT and found a level of noise which is smaller than the minimum expected. Another finding is the very low level of 1/f noise, which is significantly lower than previous observations. We propose two possible interpretations for this strong shot noise reduction: i) Transport within a rope takes place through a few nearly ballistic tubes within a rope and possibly involves non integer effective charges. ii) A substantial fraction of the tubes conduct with a strong reduction of effective charge (by more than a factor 50).Comment: Submitted to Eur. Phys. J. B (Jan. 2002) Higher resolution pictures are posted on http://www.lps.u-psud.fr/Collectif/gr_07/publications.htm

Environmental effects in the third moment of voltage fluctuations in a tunnel junction

We present the first measurements of the third moment of the voltage fluctuations in a conductor. This technique can provide new and complementary information on the electronic transport in conducting systems. The measurement was performed on non-superconducting tunnel junctions as a function of voltage bias, for various temperatures and bandwidths up to 1GHz. The data demonstrate the significant effect of the electromagnetic environment of the sample.Comment: Major revision. More experimental results. New interpretation. 4 pages, 3 figure

Turbulence Model Comparison for Compact Plate Heat Exchanger Design Application.

In the framework of the Gas-Power Conversion System for the Advanced Sodium Technological Reactor for Industrial Demonstration (ASTRID) project design, works done at CEA are focused on the design of the sodium-gas heat exchanger. Compact plate heat exchangers are indicated as the most suitable technology for such applications. An innovative compact heat exchanger geometry is proposed in this paper: its innovationconsists increatinga 3D mixing flow. The proposed geometry has also very good mechanical resistance to high pressure gradients, being suitable for a large variety of flow applications. The flowfield inside such a channel is experimentally studied using the Laser Doppler Velocimetry (LDV) technique. The main velocity, the radial velocity as well as the Reynolds stressesare measured: data show the high level of flow mixing and the 3D flow pattern inside the channel. The experimental measurements are then used to validate turbulence models: in particular Reynolds-Averaged Navier-Stokes (RANS) equations are closed using both isotropic 2-equation isotropic eddy viscosity models and a Non-Linear Eddy Viscosity Model (NLEVM). Presented results represent the first step in the assessment of innovative high-performance compact plate heat exchangers that can be used to increase the plant efficiency as well as decrease the capital cost of the single component

Conductance of Aharonov--Bohm Rings: From the Discrete to the Continuous Spectrum Limit

The dissipative conductance of an array of mesoscopic rings, subject to an a.c. magnetic flux is investigated. The magneto--conductance may change sign between canonical and grand-canonical statistical ensembles, as function of the inelastic level broadening and as function of the temperature. Differences between canonical and grand-canonical ensembles persist up to temperature of the order of the Thouless energy.Comment: 13 pages, 2 figures, REVTeX v2.1, WIS--93/121/Dec.--P

Sign Reversals of ac Magnetoconductance in Isolated Quantum Dots

We have measured the electromagnetic response of micron-size isolated mesoscopic GaAs/GaAlAs square dots down to temperature T=16mK, by coupling them to an electromagnetic micro-resonator. Both dissipative and non dissipative responses exhibit a large magnetic field dependent quantum correction, with a characteristic flux scale which corresponds to a flux quantum in a dot. The real (dissipative) magnetoconductance changes sign as a function of frequency for low enough density of electrons. The signal observed at frequency below the mean level spacing corresponds to a negative magnetoconductance, which is opposite to the weak localization seen in connected systems, and becomes positive at higher frequency. We propose an interpretation of this phenomenon in relation to fundamental properties of energy level spacing statistics in the dots.Comment: 4 pages, 4 eps figure

Experimental characterisation of the convective heat transfer in a vortex -wall interaction

Abstract The development of turbulence models and wall laws for the numerical simulation of flows in complex geometries requires a detailed experimental analysis of turbulence and of the phenomena that appear in turbulent boundary layers. There is a strong need to develop new measurement systems allowing the determination of unsteady wall heat transfer coefficients. In order to improve the knowledge of the unsteady phenomena occurring in perturbed boundary layers, a fundamental study is conducted on the interaction of a single vortex with a flat plate. An experimental methodology using a specific thermal sensor whose surface temperature is measured by an infrared thermography system is presented. It allows the characterisation of the unsteady convective heat transfer coefficient whose evolution is compared with the fluctuations of the wall friction coefficient, calculated from velocity profiles measured by laser Doppler velocimetry

Dynamic response of isolated Aharonov-Bohm rings coupled to an electromagnetic resonator

We have measured the flux dependence of both real and imaginary conductance of $GaAs/GaAlAs$ isolated mesoscopic rings at 310 MHz. The rings are coupled to a highly sensitive electromagnetic superconducting micro-resonator and lead to a perturbation of the resonance frequency and quality factor. This experiment provides a new tool for the investigation of the conductance of mesoscopic systems without any connection to invasive probes. It can be compared with recent theoretical predictions emphasizing the differences between isolated and connected geometries and the relation between ac conductance and persistent currents. We observe $\Phi_0/2$ periodic oscillations on both components of the magnetoconductance. The oscillations of the imaginary conductance whose sign corresponds to diamagnetism in zero field, are 3 times larger than the Drude conductance $G_0$. The real part of the periodic magnetoconductance is of the order of $0.2 G_0$ and is apparently negative in low field. It is thus notably different from the weak localisation oscillations observed in connected rings, which are much smaller and opposite in sign.Comment: 4 pages, revtex, epsf, 4 Postscript file

Magneto-polarisability of mesoscopic systems

In order to understand how screening is modified by electronic interferences in a mesoscopic isolated system, we have computed both analytically and numerically the average thermodynamic and time dependent polarisabilities of two dimensional mesoscopic samples in the presence of an Aharonov-Bohm flux. Two geometries have been considered: rings and squares. Mesoscopic correction to screening are taken into account in a self consistent way, using the response function formalism. The role of the statistical ensemble (canonical and grand canonical), disorder and frequency have been investigated. We have also computed first order corrections to the polarisability due to electron-electron interactions. Our main results concern the diffusive regime. In the canonical ensemble, there is no flux dependence polarisability when the frequency is smaller than the level spacing. On the other hand, in the grand canonical ensemble for frequencies larger than the mean broadening of the energy levels (but still small compared to the level spacing), the polarisability oscillates with flux, with the periodicity $h/2e$. The order of magnitude of the effect is given by $\delta \alpha/\alpha \propto (\lambda_s/Wg)$, where $\lambda$ is the Thomas Fermi screening length, $W$ the width of the rings or the size of the squares and $g$ their average dimensionless conductance. This magnetopolarisability of Aharonov-Bohm rings has been recently measured experimentally \cite{PRL_deblock00} and is in good agreement with our grand canonical result.Comment: 12 pages, 10 figures, revte

Diamagnetic orbital response of mesoscopic silver rings

We report measurements of the flux-dependent orbital magnetic susceptibility of an ensemble of 10^5 disconnected silver rings at 217 MHz. Because of the strong spin-orbit scattering rate in silver this experiment is a test of existing theories on orbital magnetism. Below 100 mK the rings exhibit a magnetic signal with a flux periodicity of h/2 e consistent with averaged persistent currents, whose amplitude is estimated to be of the order of 0.3 nA. The sign of the oscillations indicates diamagnetism in the vicinity of zero magnetic field. This sign is not consistent with theoretical predictions for average persistent currents unless considering attractive interactions in silver. We propose an alternative interpretation taking into account spin orbit scattering and finite frequency.Comment: 4 pages, 4 figures, revtex4, accepted for publication in Physical Review Letter

Magneto-polarisability of mesoscopic rings

We calculate the average polarisability of two dimensional mesoscopic rings in the presence of an Aharonov-Bohm flux. The screening is taken into account self-consistently within a mean-field approximation. We investigate the effects of statistical ensemble, finite frequency and disorder. We emphasize geometrical effects which make the observation of field dependent polarisability much more favourable on rings than on disks or spheres of comparable radius. The ratio of the flux dependent to the flux independent part is estimated for typical GaAs rings.Comment: pages, Revtex, 1 eps figur